During the 2012 soybean growing season, soybean (Glycine max (L.) Merr.) plants submitted to South Dakota State University Plant Diagnostic Clinic exhibited symptoms typical of sudden death syndrome (SDS) caused by Fusarium virguliforme (Aoki, O'Donnell, Homma, & Lattanzi). In the 2013 soybean growing season, a soybean survey targeting SDS-symptomatic plants was carried out in 20 eastern South Dakota counties between July and August when plants were at the beginning seed and beginning seed maturity growth stages. Soybean plants with SDS-like symptoms were found in eight counties at very low incidence (<3%). Approximately 15 plants per field that had symptoms resembling those of SDS were collected and fungal isolations were made. Leaf symptoms included some necrosis and slight interveinal chlorosis. The tap roots also had areas of necrosis and the vascular system was brown. Isolations were made from the symptomatic tap root sections. The tap root sections were surface sterilized using a 10% NaOCl for 1 min and then rinsed once for 1 min with sterile water before being placed on an acidified potato dextrose agar. Slow growing isolates of F. virguliforme with characteristic blue sporodochia were isolated from these symptomatic plant roots. The conidia were banana-shaped with 4 to 5 septae, a typical characteristic for F. virguliforme. Koch's postulates were performed using a modified layer test method (2). Briefly, the conidia from the isolate (PL1200158 from Yankton county, SD) was used to infest sterile sorghum seed. In the greenhouse, three holes were punched in the bottom of 32 oz. Styrofoam cups. The bottom 11 cm of the cup was then filled with vermiculite. A 2-cm layer of fully colonized sorghum seed was placed on top of the vermiculite. This was covered with a 2- to 5-cm layer of vermiculite. Fifteen soybean cv. Sloan seeds were placed on top of this vermiculite layer and covered with approximately 2 cm more vermiculite for each cup for a total of 12 cups. The temperature in the greenhouse was approximately 23°C with 14 h of light and 10 h of darkness for 21 days. Leaves began to show necrosis and the roots had brown, rotted lesions. Symptoms did not develop on non-inoculated controls. After 5 weeks under greenhouse conditions, the roots of infected plants were removed, surface sterilized, and F. virguliforme was re-isolated. SDS was further confirmed by PCR using primers designed from FvTox1 gene. FvTox1, a single-copy gene, has been found to be highly species specific and primers from this region delineate F. virguliforme from other Fusarium species (1). The PCR product size matched that of expected size. The PCR product was sequenced and a BLAST search matched (100%) only the sequences of F. virguliforme FvTox1 gene (GenBank Accession No. JF440964). The confirmation of SDS in eight counties in South Dakota indicates that SDS may be widespread and a concern for soybean production when conditions are conducive for SDS to develop.
References: (1) G. C. Y. Mbofung, et al. Plant Dis. 95:1420, 2011. (2) A. F. Schmitthenner and R. G. Bhatt. Useful Methods for Studying Phytophthora in the Laboratory. Special Circular, Ohio Agricultural Research and Development Center, Wooster, OH, 1994.
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